1. Technical Field
The present invention relates to acoustic amplifiers, and more particularly to an external-magnet-type magnetic circuit that uses a composite magnet with inner diameter alignment, and provides adequate magnetic flux density in limited volume.
2. Description of Related Art
According to ways of energy conversion, amplifiers are generally classified into the dynamic type, the electromagnetic type and the electric condenser type. Therein a dynamic-type amplifier, also known as a moving-coil-type amplifier, mainly works using the electromagnetic induction generated between its internal magnetic circuit and voice coil to make a diaphragm vibrate. The magnet in the magnetic circuit plays an important role as it serves to provide a uniform and stable magnetic field for the vibration of the voice coil.
Furthermore, magnetic circuits in amplifiers can be divided into internal magnetic circuits and external-magnet-type magnetic circuits according to where the magnet is located in the magnetic circuit. Due to difficulty in magnet manufacturing, small-size amplifiers typically adopt an external-magnet-type magnetic circuit. FIG. 1 depicts an external-magnet-type magnetic circuit 1, which comprises a magnetism-guiding yoke 2. The magnetism-guiding yoke 2 has a plate portion 2a and a core portion 2b raised from the inner portion of the top surface of the plate portion 2a, so that it has an L-like sectional shape. A permanent magnet 3 is deposited on the outer portion of the top surface of the plate portion 2a. A magnetism-guiding plate 4 is installed on the top surface of the permanent magnet 3 and is separated from the core portion 2b by a magnetic gap 5. Therein, the two magnetic poles of the permanent magnet 3 are at two ends and face the plate portion 2a of the magnetism-guiding yoke 2 and the magnetism-guiding plate 4, respectively, thereby guiding the magnetic lines of force into the magnetic gap 5 to form a uniform magnetic field. For maintaining adequate magnetic flux density, the permanent magnet 3 would have to take a certain space and this is against to the trend of miniaturization. In addition, magnetic flux leakage tends to happen in such a conventional external-magnet-type magnetic circuit. Hence, the prior-art structure needs to be improved in terms of both size and capability of allowing good acoustic performance of amplifiers using it.